Replace flip parameter in erode/dilate with shift_x and shift_y

This change allows morphological open/close and white/black tophat to correctly use rectangular structuring element that has one even-numbered dimension. Note: dilate and erode functions flip the definitions of x/y and width/height, but it does so consistently so it doesn't really matter.

scikits/image/morphology/cmorph.pyx

@@ -23,12 +23,13 @@ cdef inline int int_min(int a, int b): return a if a <= b else b
 def dilate(np.ndarray[IMAGE_DTYPE_t, ndim=2] image not None,
            np.ndarray[IMAGE_DTYPE_t, ndim=2] selem not None,
            np.ndarray[IMAGE_DTYPE_t, ndim=2] out,
-           bool flip):
+           bool shift_x, bool shift_y):
     cdef int hw = selem.shape[0] // 2
     cdef int hh = selem.shape[1] // 2
-    if flip:
-        hw -= 1
+    if shift_x:
         hh -= 1
+    if shift_y:
+        hw -= 1
 
     cdef int width = image.shape[0], height = image.shape[1]
     if out is None:
@@ -71,12 +72,13 @@ def dilate(np.ndarray[IMAGE_DTYPE_t, ndim=2] image not None,
 def erode(np.ndarray[IMAGE_DTYPE_t, ndim=2] image not None,
           np.ndarray[IMAGE_DTYPE_t, ndim=2] selem not None,
           np.ndarray[IMAGE_DTYPE_t, ndim=2] out,
-          bool flip):
+          bool shift_x, bool shift_y):
     cdef int hw = selem.shape[0] // 2
     cdef int hh = selem.shape[1] // 2
-    if flip:
-        hw -= 1
+    if shift_x:
         hh -= 1
+    if shift_y:
+        hw -= 1
 
     cdef int width = image.shape[0], height = image.shape[1]
     if out is None:

scikits/image/morphology/grey.py

@@ -9,7 +9,7 @@ import numpy as np
 
 eps = np.finfo(float).eps
 
-def greyscale_erode(image, selem, out=None, flip=False):
+def greyscale_erode(image, selem, out=None, shift_x=False, shift_y=False):
     """
     Performs a greyscale morphological erosion on an image given a flat
     structuring element. The eroded pixel at (i,j) is the minimum over all
@@ -27,8 +27,8 @@ def greyscale_erode(image, selem, out=None, flip=False):
        The array to store the result of the morphology. If None is
        passed, a new array will be allocated.
 
-    flip : bool
-        Flip structuring element about center point. This only affects
+    shift_x, shift_y : bool
+        shift structuring element about center point. This only affects
         eccentric structuring elements (i.e. selem with even numbered sides).
 
     Returns
@@ -40,12 +40,13 @@ def greyscale_erode(image, selem, out=None, flip=False):
         raise NotImplementedError("In-place erosion not supported!")
     try:
         import scikits.image.morphology.cmorph as cmorph
-        out = cmorph.erode(image, selem, out=out, flip=flip)
+        out = cmorph.erode(image, selem, out=out,
+                           shift_x=shift_x, shift_y=shift_y)
         return out;
     except ImportError:
         raise ImportError("cmorph extension not available.")
 
-def greyscale_dilate(image, selem, out=None, flip=False):
+def greyscale_dilate(image, selem, out=None, shift_x=False, shift_y=False):
     """
     Performs a greyscale morphological dilation on an image given a flat
     structuring element. The dilated pixel at (i,j) is the maximum over all
@@ -64,8 +65,8 @@ def greyscale_dilate(image, selem, out=None, flip=False):
        The array to store the result of the morphology. If None, is
        passed, a new array will be allocated.
 
-    flip : bool
-        Flip structuring element about center point. This only affects
+    shift_x, shift_y : bool
+        shift structuring element about center point. This only affects
         eccentric structuring elements (i.e. selem with even numbered sides).
 
     Returns
@@ -77,7 +78,8 @@ def greyscale_dilate(image, selem, out=None, flip=False):
         raise NotImplementedError("In-place dilation not supported!")
     try:
         from . import cmorph
-        out = cmorph.dilate(image, selem, out=out, flip=flip)
+        out = cmorph.dilate(image, selem, out=out,
+                            shift_x=shift_x, shift_y=shift_y)
         return out;
     except ImportError:
         raise ImportError("cmorph extension not available.")
@@ -106,12 +108,12 @@ def greyscale_open(image, selem, out=None):
        The result of the morphological opening.
     """
     h, w = selem.shape
-    if (h % 2) == 0 and (w % 2) == 0:
-        flip = True
-    else:
-        flip = False
+    shift_x = True if (w % 2) == 0 else False
+    shift_y = True if (h % 2) == 0 else False
+
     eroded = greyscale_erode(image, selem)
-    out = greyscale_dilate(eroded, selem, out=out, flip=flip)
+    out = greyscale_dilate(eroded, selem, out=out,
+                           shift_x=shift_x, shift_y=shift_y)
     return out
 
 def greyscale_close(image, selem, out=None):
@@ -137,12 +139,12 @@ def greyscale_close(image, selem, out=None):
        The result of the morphological opening.
     """
     h, w = selem.shape
-    if (h % 2) == 0 and (w % 2) == 0:
-        flip = True
-    else:
-        flip = False
+    shift_x = True if (w % 2) == 0 else False
+    shift_y = True if (h % 2) == 0 else False
+
     dilated = greyscale_dilate(image, selem)
-    out = greyscale_erode(dilated, selem, out=out, flip=flip)
+    out = greyscale_erode(dilated, selem, out=out,
+                          shift_x=shift_x, shift_y=shift_y)
     return out
 
 def greyscale_white_top_hat(image, selem, out=None):